Blow molded containers such as fuel containers including gasoline tanks are manufactured by blow molding. Blow molding is a process in which a resin is molten in an extruder and extruded into a tubular parison; the parison is clamped between molds and a pressurized gas is blown into the parison through a blow pin to swell and shape the parison to match the cavity in the mold; and the resin is cooled. The blow molding process is easy and can be applied to a variety of molded articles ranging from complex shapes such as gasoline tanks and drum cans to panel-shaped articles. This process is therefore used widely in the industry.
Fuel tanks for internal combustion engines such as automobile engines have been typically made of metals. In recent years, however, there has emerged a trend to use plastic fuel tanks because of the need for automotive lightening aimed at energy saving as well as rust-freeness and easy molding to the desired shape. However, no techniques have been established with regard to plastic fuel tanks capable of sufficient and balanced characteristics even in reduced thickness, and processes for producing such tanks with good productivity. Accordingly, novel techniques are constantly desired in the plastic fuel tank industry.
In general, plastic fuel tanks typically have a large volume of approximately 60 liters. To meet this volume requirement while corresponding to the need of thickness reduction that is expected to grow progressively in the future, very high material properties will be required for plastic fuel tanks. In detail, the fuel tanks will be required to have good environmental stress crack resistance, creep resistance and impact resistance.
Patent Document 1 discloses polyethylene compositions suited for the blow molding of gasoline tanks. The composition is a blend of a high molecular weight polymer and a low molecular weight polymer and is produced with a Ziegler catalyst. However, it is conceivable that the compositions cannot meet the aforesaid property requirements. In the working examples of Patent Documents 2 and 3, fuel tanks are disclosed which are produced from titanium catalyst-catalyzed ethylene polymers and show excellent impact resistance even in small thickness. Patent Documents 4 and 5 disclose multimodal polyethylene molding materials produced with a titanium catalyst and improved in ESCR-rigidity balance and swelling ratio, and also disclose blow molded articles such as fuel tanks produced from the blow molding materials.
Patent Document 1: JP-A-H06-172594
Patent Document 2: JP-A-H07-090021
Patent Document 3: JP-A-H07-101433
Patent Document 4: JP-A-2003-510429
Patent Document 5: JP-A-2006-193671
Patent Document 6: JP-A-2005-523968
Patent Document 7: WO 2004/083265
Patent Document 8: WO 2006/019147